Physicists created an electron 'catapult' that moves particles at 'extraordinary' speed

Molecular vibrations can "catapult" electrons across solar materials in quadrillionths of a second‬ ‪— much faster than previously thought, a new study shows.

The findings could help scientists find more efficient ways to convert solar energy into electricity, according to the study, which was published March 5 in the journal Nature Communications.

"We're effectively watching electrons migrate on the same clock as the atoms themselves," study co-author Pratyush Ghosh, a researcher who studies ultrafast spectroscopy at the University of Cambridge, said in a statement.

Organic molecules go solar

Organic solar cells use carbon-based molecules, rather than silicon, to convert sunlight into electricity. In theory, organic solar cells could provide that electricity at lower cost than conventional solar cells, but they are much less efficient.

In a typical organic solar cell, an electron donor and an electron acceptor are sandwiched between two conductive electrodes. When light hits the cell, it generates an "exciton," an electron-hole pair. Excitons split at the interface between the donor and the acceptor, generating electricity.

To achieve fast charge transfer at the interface and limit energy loss, the donor and acceptor molecules usually have strong electronic coupling, or overlap between their electronic states, which allows charges to move easily between molecules. They also often have a large energy difference between them, but that limits the voltage available from the device.

In the new study, researchers observed ultrafast charge transfer at a junction between the electron donor and electron acceptor in an organic solar cell, without needing to conform to either of these constraints. The team used a short laser pulse to excite the electron donor, a polymer called TS-P3, and then used a different laser to measure how the system changed during charge transfer.

That charge transfer happened in 18 femtoseconds ‪—‬ about as fast as an individual molecule vibrates. A few other systems without strong driving forces exhibit charge transfer over 100 to 200 femtoseconds, but most take ten to a thousand times that long.

"Seeing it happen on this timescale within a single molecular vibration is extraordinary," Ghosh said in the statement.

A 'molecular catapult'

That similar timescale wasn't a coincidence. In a second set of laser experiments, the team found that vibrations in the polymer donor molecule launched an electron across the junction to an acceptor molecule. When the electron arrived, it triggered overlapping vibrations in the acceptor molecule. This overlap allowed charge transfer to happen much more quickly than expected, and without the need for strong coupling or a large energy difference.

"Instead of drifting randomly, the electron is launched in one coherent burst," Ghosh said in the statement. "The vibration acts like a molecular catapult. The vibrations don't just accompany the process, they actively drive it."

The findings help to explain the processes that control the speed of charge transfer and establish new strategies for designing more efficient organic solar cells and materials, the researchers wrote in the study.

"Instead of trying to suppress molecular motion, we can now design materials that use it ‪—‬ turning vibrations from a limitation into a tool," study co-author Akshay Rao, a physicist at Cambridge, said in the statement.